A packaging laminate is previously known from, for example, international Patent Application carrying Publication Number WO97/02140. The prior art packaging laminate displays a rigid, but foldable core or bulk layer of paper or paperboard and outer, liquid-tight coatings of moisture- and heat resistant thermoplastic material on both sides of the core layer. In order to impart to the prior art packaging laminate tightness properties also against gases, in particular oxygen gas, the packaging laminate moreover displays a gas barrier, e.g. an aluminium foil, disposed between the core layer and the one outer coating.
From, for example, a flat-folded tubular packaging blank, retortable packaging containers are produced in that the packaging blank is first raised to an open, tubular packaging carton which is sealed at its one end by fold forming and sealing of the continuous, foldable end panels of the packaging carton for the formation of a substantially planar bottom seal. The packaging carton provided with the bottom is filled with the relevant contents, for example a food, through its open end, which is thereafter sealed by an additional fold-forming and sealing of the corresponding end panels of the packaging carton for the formation of a substantially planar top seal. Naturally, the top seal of the packaging carton may instead be formed first, in which event filling is instead put into effect via the bottom. The filled and sealed, normally parallelepipedic, packaging container is thereafter ready for a heat treatment in order to impart to the packed food extended shelf-life in the unopened packaging container, for example a shelf-life of at least 6 months, often even longer, for instance at least 12 or 18 months.
A heat treatment of the packed food with the intention of extending its shelf-life may suitably be put into effect by the method and under the conditions described in greater detail in international Patent Application carrying Publication Number WO98/16431, which is hereby incorporated as reference. The packaging container is placed in a retort and heated therein with the aid of a first circulating gaseous medium, for example hot steam, to a temperature which in general lies within the range of between 70 and 130° C. After a predetermined stay-time at the selected temperature, the supply of the first gaseous medium is discontinued. Thereafter, the packaging container is cooled by means of a second circulating gaseous medium, for example cold air, and finally by means of a circulating liquid medium, for example cold water. The cooled packaging container is thereafter removed from the retort for further transport and handling.
One variation of such retorting is so-called hot fill which is particularly employed for acidic products. In such instance, a sterile filling of the product takes place at at least 80° C. but below 100° C., whereafter no retorting is required.
Since, for example, soups, sauces and other ready-to-consume dishes may be packed in such a packaging container, there is a manifest need in the art for the contents to be able to be heated in a microwave oven when it is still in the opened packaging container. The problem in being able to achieve this is naturally that a gas barrier layer of aluminium jams the microwave radiation, for which reason a packaging container with such a gas barrier layer cannot be used in a microwave oven. Granted, it is known in the art that other gas barrier layers may be employed, such as polymer-based layers, which satisfy the general requirements placed on barrier properties. However, a major drawback in such polymer-based gas barrier layers is that they do not permit thermosealing by induction, which is afforded by gas barrier layers of aluminium or other electrically conductive materials. Induction thermosealing is a well tested and well functioning method which gives extremely tight seals. The superior adhesion which is attained in such seals moreover makes for an extremely good openability for an opening indication in the form of, for example, a perforation for tearing, in other words without the need of scissors. Further, the aluminium layer is almost a necessity if such perforations are to be able to be realised using laser perforation methods on the finished laminate and only partly through its thickness, in which event the laser perforation beam is arrested by the aluminium layer so that this layer and the layer or layers lying inside the aluminium layer are not perforated.
To sum up, it would thus be desirable to still be able to employ aluminium which, hence, is a need which runs contrary to the need for a microwave-friendly packaging material. Moreover, it is desirable from the viewpoint of the environment to be able to reduce the employment of aluminium in packaging materials.
It is previously known from EP 0 378 880 B1 to provide a paper-based packaging laminate with strips of aluminium whose purpose is to permit thermosealing by means of induction in the regions thereof. However, the publication is silent as to whether the quantity of aluminium would be sufficiently slight for the laminate to be microwave-friendly or whether the laminate can give a packaging container which permits retorting or hot fill. Nor is there any disclosure made concerning any opening arrangement or indication on the packaging laminate or packaging containers produced therefrom. Further, the method which is described for applying the aluminium strips on the laminate suffers from many drawbacks. The reason for this is that it is an intermittent method in which short strips are applied across the width of the laminate web with uniform spacing which corresponds to the positions where transverse seals are later made in the same line, in a continuous forming-filling-sealing process on the basis of the laminate which is continuously formed into a tube, filled with the intended contents, transversely sealed through the contents and thereafter cut into cushion-like containers which are subsequently fold-formed into, for example, brick shape. That this is a matter of an intermittent method in itself affords disadvantages as regards, for example, equipment, adjustment and operation. Further, even if the aluminium strip is, as stated, covered by polyethylene on both sides, the edge of the strip will expose the aluminium to the product which is to be stored in the packaging container and this is not acceptable for food products.